That very strong quake struck at 10:18 p.m. PDT Sunday and was centered 50 miles west of Eureka and about four miles beneath the Pacific seabed, according to the U.S. Geological Survey. It was followed by about a half-dozen aftershocks, including one of magnitude 4.6.

The 6.8 earthquake off the coast of Eureka has made scientists postulate that California's long awaited "Big One" may not come from the San Andreas fault, as was previously thought, but from the lesser-known Cascadia fault.

The Cascadia fault produced last Sunday's earthquake and well as a 7.2 earthquake off Crescent City in 2005, the two largest California earthquakes in the last decade. Representatives of the California Geological Survey told the Los Angeles Times that the fault has caused six earthquakes of 7.0 of more in the last century.